In my speculative theology, both Saint John the Baptist and Saint Joseph, though conceived with original sin, received the special grace and privilege to be able to avoid all personal sin. Jesus indicated this truth when He described John in this way:

[Luke]
{7:26} Then what did you go out to see? A prophet? Certainly, I tell you, and more than a prophet.
{7:27} This is he of whom it is written: “Behold, I send my Angel before your face, who shall prepare your way before you.”
{7:28} For I say to you, among those born of women, no one is greater than the prophet John the Baptist. But he who is least in the kingdom of God is greater than he.”

Jesus taught that John the Baptist is more than a prophet, and that he is like a holy Angel. But the holy Angels of God are all sinless. Moreover, by saying that no one born of women is greater than John, Jesus implied that no Saint, past, present, or future, is greater than John. For this to be the case, John would have to have fewer sins than any other Saint, or no sins at all.

Blessed Anne Catherine Emmerich spoke of John the Baptist, based on her visions from God.

“He sees, he knows, he speaks only Jesus…. In the desert, blameless and pure as a babe in the mother’s womb, he comes forth from his solitude innocent and spotless as a child at the mother’s breast. ‘He is pure as an angel,’ I heard the Lord say to the Apostles. ‘Never has impurity entered into his mouth, still less has an untruth or any other sin issued from it.’ “

According to Blessed Anne Catherine, Jesus taught the Apostles about the great holiness of John the Baptist. To avoid speaking any untruth and any other kind of spoken sin is to be perfect (James 3:2). And to be pure as an Angel, one would have to be sinless. Jesus therefore implied that John the Baptist is sinless.

More in my book, New Insights into the Deposit of Faith.